Nuclear magnetic resonance (NMR) spectrometers first became available in 1946. In 1950 observations of "shifted" resonance in nitrogen spectra by W. G. Proctor & F. C. Yu, Phys. Rev. 77, 717, (1950) stimulated efforts to improve the homogeneity and stability of magnets used in the experiments and led to the observation of chemically shifted resonances in proton spectra by J. T. Arnold, S. S. Dharmatti, and M. E. Packard, Jour. Chem. Phys. 19, 1608, (1951). This marked the beginning of high resolution NMR and its application as an analytical tool for chemistry, and sparked rapid growth in the development of NMR spectrometers. This development continues today at a pace limited only by the availability of relevant technology. The present work is predicated upon improvements in rf probe performance incorporating receiver coils and other parts made from recently available high temperature superconducting (HTS) materials. HTS materials are type II superconductors. The terms "HTS materials" and "type II superconductors" will hereafter be used interchangeably herein.
Nuclei of most isotopes of the elements have non-zero spin and exhibit gyromagnetic properties. These non-zero spin nuclei behave like microscopic spinning bar magnets. When a static homogeneous magnetic field B is applied to an ensemble of spin active nuclei, the spins align, some in the direction of the field and some in the direction opposed to the field. A net polarization of the ensemble of spins in the direction of the field results and the spins are said to be "polarized" by the field. If a polarized ensemble of nuclei is simultaneously subjected to an rf magnetic field, usually called the B.sub.1 field, said B.sub.1 field having an appropriate frequency and spatial orientation with respect to the polarizing field B, an NMR response signal can be generated.
The broad general utility of NMR as a tool for determining the chemical structure of compounds is due to the influence of the molecular environment on the local magnetic field at the nuclei. The local magnetic field at the nucleus of a particular nuclear species at a particular site in a molecule is the vector addition of the externally applied field and the field caused by the magnetic influence of its molecular environment. By way of example, circulation of electrons about the nucleus caused by the applied field results in an induced field at the nucleus which in some instances opposes the applied field (diamagnetism), and in some instances augments it (paramagnetism). By way of further example the local field at a nucleus can be additionally modified, taking on multiple values or "splitting" due to interactions with other spin active nuclei in the molecule. These two effects, known as "chemical shift" and "spin-spin coupling" respectively, are major sources of the fine structure seen in NMR spectra as more fully described in "Introduction To NMR Spectroscopy", R. Abrahms, J Fisher, P. Loftus, J. Wiley & Sons, 1993, chap. 2, pp. 13-33, chap. 3, pp. 34-59. NMR spectra which are characterized by resonance lines that are narrower than the shifts in resonance caused by chemical shift and spin-spin coupling are known as high resolution spectra. These lines are primarily made possible to observe by the application of an extremely homogeneous polarizing field. The frequency of the NMR response signal is proportional to the local magnetic field at the nuclei, the proportionality constant being .gamma., the magnetogyric ratio. Any slight deviation from homogeneity of the local magnetic field over the sample region causes a corresponding shift in the resonance of the nuclei affected resulting in undesirable line broadening of the response signal.
An NMR spectrometer is comprised of: 1) a DC magnet which provides the stable, homogeneous, static magnetic field required for polarizing the spins, 2) an rf system which provides a suitable rf excitation signal, 3) a coil or a plurality of coils for coupling the rf excitation to the spins and for receiving the NMR response signal, 4) a detection system for detecting the NMR response signal, 5) a signal processing system for processing the detected NMR response signal, and 6) an output device for displaying the NoM response signal. For high resolution NMR studies, the compound under investigation is usually dissolved in or mixed with a suitable solvent and is in liquid form contained in a sample tube which is typically 5 mm in diameter. The apparatus known as the probe holds the sample in a sample holder portion of a probe in the most homogeneous region in the magnetic field. The coil or coils for coupling the rf excitation to the sample and for detecting the NMR response signal are also mounted to the probe.
NMR is an inherently insensitive technique. Sensitivity is strictly defined in terms of the minimum concentration of a test material required to produce a signal that is just detectable above the level of noise. For practical purposes however, the signal to noise ratio, S/N, is generally considered a good measure of sensitivity. Continued improvement in sensitivity has been a constant objective in the development of NMR spectrometers. Increasing signal strength, reducing noise, and improving signal processing methods have all contributed to this improvement. Many of the factors that influence the attainable signal to noise ratio are treated in "A Handbook of Nuclear Magnetic Resonance", R. Freeman, Longman Scientific & Technical, 1988, pp. 216-229 which is hereby incorporated herein by reference.
In addition to sensitivity, resolution of spectral information is an important aspect of NMR spectrometer performance. Natural line widths can be narrow for liquid samples, less than 1 Hz by way of example. To avoid degrading the resolution, both the static magnetic field B and the rf excitation field B.sub.1 should be homogeneous over the volume of the sample, and stable over the time of data acquisition to the order of 1 part in 10.sup.9. The data acquisition time can be very long, particularly when acquiring the spectra of nuclei other than protons, such as .sup.13 C by way of example. For .sup.13 C nuclei using natural abundance samples, the overall sensitivity relative to .sup.1 H is 1.7.times.10.sup.-4. The direct observation of .sup.13 C nuclei therefore typically requires many scans and may require averaging the NMR responses over hours or days to achieve the required signal to noise ratio. Any small change in the magnetic field over this time period will cause the NMR signal to shift slightly and effectively broaden the resonance response. Field homogeneity requirements are addressed by careful magnet design, the use of shimming coils and by spinning the sample. Field-frequency lock systems, such as described in "Modern NMR Spectroscopy", J. K. M. Sanders & B. K. Hunter, Oxford University Press, 1993, chap. 1, pp. 39-41. are used to achieve the required stability.
The probe is a critical component in an NMR spectrometer. For a given static magnetic field strength and a given sample size, the performance of the probe largely determines the sensitivity of the spectrometer. An important consideration in probe design is the coupling efficiency .zeta. of the receiver coil to the sample. .zeta. is the ratio of effective inductance to total inductance of the receiver coil. Any portion of the inductance of the receiver coil that does not contribute towards the detection of the NMR signal, such as the inductance of the leads of the receiver coil by way of example, results in a loss of sensitivity proportional to .zeta..sup.1/2. Another important consideration is the quality factor Q of the receiver coil which affects sensitivity by a factor of Q.sup.1/2, since signal voltage is proportional to Q and noise voltage is proportional to Q.sup.1/2. Q represents the ratio of energy stored in the receiver coil resonant circuit to the energy dissipated through resistive losses in the circuit. Another important consideration in probe design is the receiver-coil filling factor .xi. which, for a fixed coil volume, influences the signal strength and the sensitivity directly. .xi. is a measure of the energy stored in the transverse magnetic field coupling to the sample, compared to the total magnetic energy stored in the receiver coil resonant circuit. Filling factor .xi., coupling efficiency .zeta., and quality factor Q should all be as large as possible for maximum sensitivity.
Modern spectrometers use superconducting DC magnets for producing the static polarizing field. The sample is placed in a cylindrical tube positioned coaxial with the DC magnet. Transmitter and receiver coils made of normal, i.e. non-superconducting, materials can be saddle coils as shown in FIG. 1a or split formed-wire coils as shown in FIG. 1b. Either are ordinarily shaped to couple closely to the sample while providing the radio frequency B.sub.1 field orthogonal to the static field. Coils made of high temperature superconducting (HTS) films are very attractive for use in NMR spectrometers because of their low rf resistance and resulting low noise. Using HTS materials, coils have been fabricated by depositing a thin layer of superconductor on a flat substrate. A pair of such coils forming a magnetically coupled system known as a Helmholtz pair, placed on opposite sides of a sample, is shown in FIG. 2a. A second pair of similar HTS coils can be positioned orthogonal to the first pair as shown in FIG. 2b to provide for a field-frequency lock signal.
Best results are obtained with HTS coils when the superconductor is lattice matched to the substrate, i.e. grown epitaxially. The substrate should be a thermally conductive material to facilitate cooling of the coil and should have low magnetic susceptibility to avoid degrading the homogeneity of the magnetic field. Acceptable substrate materials include sapphire, lanthanum aluminate, and magnesium oxide. A preferred HTS material is YBa.sub.2 Cu.sub.3 O.sub.7-.delta. (YBCO), which has a critical transition temperature T.sub.C of approximately 87.degree. K. A coil made of this material is described in "HTS Receiver Coils For Magnetic Resonance Instruments", R. S. Withers, B. F. Cole, M. E. Johansson, G. C. Laing, G. Zaharchuk, Proc. SPIE, 2156, 27-35, (1994). Another Class II superconductive material useful in this coil application is Tl.sub.2 Ba.sub.2 CaCu.sub.2 O.sub.8.
For proper performance HTS coils must be maintained at a temperature significantly below their superconducting transition temperature T.sub.C. U.S. Pat. No. 5,508,613 entitled Apparatus For Cooling AMR Coils, to Vincent Kotsubo and Robert D. Black, describes an apparatus for cooling HTS coils as required for proper operation. A particular embodiment incorporates a Joule-Thomson or Cillord-McMahon closed cycle refrigeration unit which cools the coils to 25.degree. K. The coils are generally thermally isolated from the samples in this apparatus and the samples can be maintained at or near room temperature if desired.
High resolution NMR probes using HTS coils can provide higher sensitivity than probes with non-superconducting coils. For a given sample volume the sensitivity of a coil is proportional to (.xi.Q/T).sup.1/2, where T is the coil temperature and .xi. and Q are the aforementioned filling factor and quality factor respectively. A superconducting coil may have a Q of 20,000 as compared with a Q of 250 for a room temperature coil and is typically operated at 25.degree. K. as compared with 300.degree. K. for a room temperature coil. With the geometry appropriate for a 5 mm. sample tube, and allowing for the loss of filling factor required for thermal isolation of the sample from the coil, the potential sensitivity gain can approach a factor of 10.
It is known in the art that the probe materials and sample materials can cause significant distortion of the polarizing and rf magnetic fields due to their susceptibility. To achieve high resolution spectra, these distortions must be controlled and/or corrected. In particular, abrupt changes in susceptibility near the sensitive sample region of the probe can cause serious degradation of the field uniformity at the sample region, which can generally be partially corrected with shim coils. The aforementioned field distortion can be minimized by using cylindrical symmetric components and positioning material boundaries as far removed a possible from the sample region. Additionally, careful choice of the materials used in the probe is of paramount importance. Materials normally used in NMR probes have diamagnetic volume susceptibilities of several parts per million.
All weakly magnetic materials can be categorized as either diamagnetic or paramagnetic. When placed in a magnetic field, a diamagnetic material tends to minimize its internal flux density within, whereas a paramagnetic material tends to increase its internal flux density within itself. In either case, the presence of a magnetic material in an externally applied magnetic field will modify the field distribution in the space proximate to it as illustrated in FIGS. 3a for diamagnetic material and 3b for paramagnetic material.
The best known characteristic of superconductors is their ability to carry a steady current without any power loss., i.e., without any associated voltage drop. Complete magnetic flux expulsion, commonly known as the Meissner effect, is a second fundamental characteristic of superconductivity. The class of superconducting materials which completely expel flux from their bulk volume, thereby maintaining a condition of zero flux density internally, are known as type I superconductors. A type I superconductor is perfectly diamagnetic. Type I superconductors are characterized by a low critical transition temperature T.sub.C and a single critical magnetic field H.sub.C (T) with a relatively small range.
A large class of materials known as type II superconductors allow flux to enter the bulk of their volume in a special way and in small quantized amounts, as described in "Foundations of Applied Superconductivity", Orlando and Delin, Addison Wesley Publishing Co., 1990, chap's 6, 7, pp. 259-391, which is hereby incorporated by reference. Type II superconductors typically have a higher critical transition temperature T.sub.C than Type I superconductors and they have two critical fields, H.sub.C1 (T) and H.sub.C2 (T). For values of H&lt;H.sub.C1, type II superconductors behave like type I superconductors and exhibit the aforementioned Meissner effect. For values of H such that H.sub.C1 .ltoreq.H.ltoreq.H.sub.C2, the type II superconductor is in a mixed or vortex state in which a finite amount of flux penetrates the interior volume of the material. Since H.sub.C1 &lt;&lt;H.sub.C2, the magnetic field range for the mixed state tends to be large over most of the superconducting temperature range. Type II superconductors therefore are practical for, and useful in, engineering applications such as NMR probe coils.
The dipole field required to account for flux expulsion can be modeled in terms of induced magnetization as illustrated in FIG. 4a to FIG. 4c. The resulting field as shown in FIG. 4c can be envisioned as the superposition of the applied magnetic field, FIG. 4a and the field created by the induced magnetization, FIG. 4b, the latter representing the intrinsic magnetic property of the material. Within the superconductor the magnetic flux density is given by B=.mu..sub.0 (H+M) where .mu..sub.0 is the permeability of free space, H is the applied field and M is the induced magnetization. Ignoring saturation effects, the relationship between the induce magnetization M and the applied field H is linear for type I superconductors. The magnetic susceptibility .sub..chi.m of the material is defined by the relationship M=.sub..chi.m H. For a type I superconductor .sub..chi.m =-1 and M=-H.
The relationship between induced magnetization M and applied field H is much more complicated for a type II superconductor than for a type I superconductor. As heretofore mentioned, flux vortices penetrate into the bulk volume of the superconductor at applied fields H.gtoreq.H.sub.C1. The type II superconducting material is constituted to provide pinning forces for the purpose of inhibiting lateral movement of the vortices when an externally driven current is passed through the material. Such vortex movement would cause undesired power losses. Because of the pinning forces however, the flux vortices, after penetrating the surface when the applied field exceeds H.sub.C1, are not uniformly distributed throughout the bulk of the superconductor in an equilibrium lattice, but instead are bunched up near the surface. As the applied field is further increased above H.sub.C1 the flux vortices are forced further into the superconductor but they remain non uniformly distributed throughout the bulk volume.
Because the magnetic flux and magnetization are non-uniformly distributed in the type II superconductor, the characteristics of the bulk material are best described in terms of average values of the fields over the volume. These are sometimes called the "thermodynamic fields" and will hereafter be so referred to herein. The thermodynamic magnetic field, thermodynamic magnetic flux density and thermodynamic magnetization will hereafter be designated as , , and respectively. They obey the relationship ##EQU1##
The so-called critical state model, which applies to type II superconductors with strong pinning, is described in the aforementioned and incorporated reference "Foundations of Applied Superconductivity", Orlando and Delin, Addison Wesley Publishing Co., 1990, pp. 374-380. In accordance with the critical state model, the dependence on the thermodynamic magnetic field of the thermodynamic flux density and the thermodynamic magnetization are shown in FIGS. 5a and 5b respectively as the thermodynamic field is first increased from zero to H.sub.max and then decreased from H.sub.max back to zero. It is of particular significance for purposes of the present work that the thermodynamic flux density and thermodynamic magnetization of the type II superconductor as functions of the thermodynamic field are history dependent., i.e., that they are hysteritic.
According to the critical state model, as an external magnetic field is applied to the superconducting material, surface currents are set up that flow in such a direction as to exclude magnetic flux from the interior of the material. However there is a limiting current density J.sub.C (H) that the superconductor can carry. The model assumes that there are only three states of current flow possible with a given magnetic field axis, one being zero current density for regions that have never felt the magnetic field. The other two are full current flow J.sub.C (H) perpendicular to the axis, but each are of opposite sense from the other depending on the sense of the electromotive force that accompanied the last local change of applied field. These, local currents contribute to the magnetization of the material and thereby influence its effective susceptibility. FIGS. 6a, 6c and 6e show the locally averaged magnetic flux density distributions in a thin film superconductor of thickness 2a for different values of an increasing applied magnetic field H. The field is oriented parallel to the surface of the superconductor. FIGS. 6b, 6d and 6f show the corresponding current density profiles. The applied field H at which the flux fully penetrates the film is known as the penetration field, which will hereafter be designated Hp herein. It can be shown for the aforementioned thin film of thickness 2a, that Hp=J.sub.C (H)(a) and that the thermodynamic magnetization is -Hp/2. At field values of Hp and above the effective susceptibility ##EQU2## is equal to -0.5 J.sub.C (H) a/H.
To maintain homogeneity of the magnetic fields in the sample region of an NMR spectrometer, the most critical probe component which must be considered is the coil, because it is generally closest to the sample region and inevitably includes some susceptibility discontinuities in its geometry. For normal coil materials, i.e. non-superconducting, coil field perturbations can be minimized by constructing the coils of materials such that the overall coil structure exhibits a low average value of magnetic susceptibility. This is accomplished by making the coils from a composite material with diamagnetic and paramagnetic components using methods such as electroplating by way of example, to produce a sandwich structure of the two types of materials. Overall high electrical conductivity is maintained for this structure. Suitable diamagnetic materials include copper, silver and gold. Suitable paramagnetic materials include aluminum, rhodium and platinum.
However, when employing HTS materials as probe coils to realize the aforementioned higher sensitivity, the use of sandwich structures of the two types of susceptibility materials as described in connection with normal materials above is not an available option. Therefore the improved sensitivity under these circumstances has generally been achievable only at the cost of degraded resolution. The tradeoff of degraded spectral resolution for improved sensitivity has heretofore limited realization of the full potential inherent in the use of HTS materials in NMR probes.